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What Should I Set the Allocation Unit Size to When Formatting?

In addition to asking for the file system you’d like to use, disk formatting tools will also ask for an “Allocation unit size”. What does this mean and what value should you select?

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

The Question

SuperUser reader Andrew Keeton is curious about what exactly he’s supposed to put in the allocation section when formatting a drive. He writes:

I’m formatting a 1TB external hard drive as NTFS. This drive is mainly meant for storing media such as music and video.

What should I choose for the allocation unit size setting? The options range from 512 bytes to 64K. Are there any guidelines that I might apply to other drive types? Should I stop poking around and just leave it at “default?”

While the default setting is usually the best choice for most users, let’s dig a little deeper.

The Answers

SuperUser contributors Jonathan and Andrew offer some insight. Jonathan writes:

If you are a “Standard User” by Microsoft’s definition, you should keep the default 4096 bytes. Basically, the allocation unit size is the block size on your hard drive when it formats NTFS. If you have lots of small files, then it’s a good idea to keep the allocation size small so your harddrive space won’t be wasted. If you have lots of large files, keeping it higher will increase the system performance by having less blocks to seek.

But again, nowadays hard drive capacity is getting higher and higher it makes small difference by choosing the right allocation size. Suggest you just keep the default.

Also keep in mind that the majority file are relatively small, larger files are large in size but small in units.

Andrew expands upon Jonathan’s answer with:

In terms of space efficiency, smaller allocation unit sizes perform better. The average space wasted per file will be half the chosen AUS. So 4K wastes 2K per file and 64K wastes 32K. However, as Jonathon points out, modern drives are massive and a little wasted space is not worth fussing over and this shouldn’t be a determining factor (unless you are on a small SSD).

Compare 4K vs 64K average case waste (32K-2K = 30K), for 10,000 files that only comes out to 300,000KB or around 300MB.

Instead think about how the OS uses space. Let’s say you have a 3K file which needs to grow 2K. With a 4K AUS the data needs to be split over two blocks – and they may not be together so you get fragmentation. With a 64K AUS there are a lot fewer blocks to keep track of and less fragmentation. 16x the block size means 1/16th the number of blocks to keep track of.

For a media disk where you photos, music and videos are stored, every file is at least 1MB I use the biggest AUS. For a windows boot partition I use the Windows default (which is 4K for any NTFS drive smaller than 16TB).

To find out what the cluster size is on an existing disk:

fsutil fsinfo ntfsinfo X:


Have something to add to the explanation? Sound off in the the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

Jason Fitzpatrick is warranty-voiding DIYer and all around geek. When he's not documenting mods and hacks he's doing his best to make sure a generation of college students graduate knowing they should put their pants on one leg at a time and go on to greatness, just like Bruce Dickinson. You can follow him on if you'd like.

  • Published 01/29/13

Comments (10)

  1. Arul

    Thanks for the useful information. I am having a 1TB HDD and i created only one partition for the whole drive. Most of the files i am storing in it is movies and pictures. Is it good to have only one partition? Or should i need to split it in multiple?

  2. Erik

    @Arul
    I believe it’s best not to split partitions if possible. Doing so only complicates things and creates unnecessary limitations; instead of one large partition, you would have 2 or more smaller ones, limiting the amount of stuff you can put on either. There isn’t much advantage to using more than a single partition, unless you want to isolate files from normal use (such as backup files or restore data). Multiple partitions are best for dual-boot setups.

  3. Thristan

    At least 2 partitions. 1 for OS/programs and another for data/documents. Imagine just having 1 partition for all then your OS goes bunkers requiring reinstall…goodbye data.

  4. Thristan

    I have my data in a separate partition and has been that way for more then 10 years now despite multiple OS reinstalls, reformat, upgrade. Only case that partition would go down is if the whole harddisk fails.

  5. Jr

    Try do not need partitions.

    I have a SSD for OS
    a HDD for Media

    Not split partitions!
    \o

  6. JimB

    Re. Allocation Unit –
    Also consider that NTFS will keep the data for (very) small files within it’s allocated space, and may also allocate several small files to the same space allocation block.
    Re partitions –
    The MFT holds details of all the files in 1 large ‘tree’ so there will come a time when the access path to data files required more of the MFT than will fit into the real memory allocated as buffering cache for the MFT, then the system needs to do a vast amount more I/O reading entries into the cache required for finding the files in a ‘folder’.
    That may be the equivalent to reading 10’s of megabytes of MFT data to do something as simple as renaming 1 file.
    You should also consider the time taken to backup, and to recover your system –
    All files in 1 partition, and a restore of the OS means restore all the data afterwards, so that means have a restorable backup of all the OS and the data.
    I’d say split the usage into
    OS partition, Your personally entered data (and email), large re-acquirable files such as multimedia, and fist-line backup of the OS and data images – remembering the common needs of recovery – malware, bad OS fix, or an error by you – as in deleting a folder instead of a file, and needing a new hard drive – so do remember to take reasonably frequent backups to external media – USB drive, stick, and occasionally DVD set.
    Now recovery is:
    Restore the OS from the OS backup – maybe 30GB of a 60 GB space.
    Restore your data – maybe 10GB of a 30GB space
    Restore the multimedia as you get to want the video’s etc.

    Because of the problem with the MFT size I also have a separate partition for small files – gif, jpg, ebooks etc.
    So the access problems associated with large (100’s of thousands) volumes of files only effects system throughput/performance whan I access that partition.
    Thristan – note the 4 partitions instead of your 2.
    BUT
    I had a system die recently – I was backing-up the data to the external USB drive – and lost the OS drive, my separate data drive, and the backup drive – I wasn’t happy at all.

  7. Dheeraj Thedijje

    @Arul no way, single partition specially for 1TB is very heavy for your computer to track all the sectors, index files on single partition will defiantly make your system slower. also as @Thristan said, one single re-installation of your OS will wipe all your data. even if you have data backed up on cloud, how silly to download 5-10 GB of them from Internet.
    Well about this article, i was 80 aware of that before but not sure, as that was my assumption, thanks for making it true for me. :)

  8. SteveMann

    On the OP question – Allocation Units are physical blocks on the hard drive. The default is 4096 bytes (4Kb). When a small file is written to the drive, it consumes the Allocation Units it needs. For example, a ten-byte file will fit into one AU. So will a 3.9Kb file. They both occupy the same amount of physical space.

    Partitioning is an anachronism that became necessary when hard-disk size was more than DOS could address and the only way to use any hard-disk over 30Gb was with partitioning.
    Today there are only a few valid reasons for partitions:
    1) Hosting more than one O/S where each O/S needs its own Boot Partition
    2) A Recovery Partition
    3) To keep the O/S in a small partition – it’s getting difficult to buy a PC with anything smaller than a 1TB boot disk, and Windows 7 needs no more than 100GB including a lot of installed applications.
    4) With 3Tb disk drives becoming available, we’ve come full-circle and have hard-disk drives larger than NTFS can address.

    Performance-wise, a separate partition is physically the same as a single disk drive. No advantage whatever.
    Two Partitions are not the same as two drives.

  9. David

    Ok, we have impact on storage. But are there any “speed” benefits to different allocation sizes? Sorry to ask another question and if yes, save it for another post.

  10. CK

    A few years ago with windows 7, I decided to try different block sizes for a media / backup internal SATA drive. The files would generally be larger than average files, with MP3 tending to be on the low end of size (5-10MB), and lots of videos & backup zip files in the 500+MB file size.

    I tried every other block size larger than 4 KB & ran actual file copies, as well as drive speed test. There was really no difference at all between difference block sizes vs. the 4KB default. I know this is just an anecdotal case. If the block size does indeed make a difference, then at the very least there was some other bottleneck or piece of the puzzle that was limiting it.

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